Modeling Mamala Bay Outfall Plumes. I: Near Field

The near-field behavior of the Sand Island, Hawaii, ocean outfall plume was modeled. The model, a modified version of the Environmental Protection Agency Roberts-Snyder-Baumgartner model, used as input data simultaneous measurements through the water column of currents obtained from Acoustic Doppler Current Profilers and density profiles obtained from thermistor strings. More than 20,000 simulations were run for a modeling period of almost one year, and frequency distributions of plume characteristics were obtained. The currents and density stratification change widely and rapidly, resulting in extreme variability in plume behavior. Rise height was predicted to vary from deeply submerged to surfacing, and near-field dilution was predicted to vary from around 100 to several thousands within a few hours. The length of the near field, or hydrodynamic mixing zone, also varies considerably, so that a fixed regulatory mixing zone may sometimes encompass all of the near field and some of the far field and sometimes only part of the near field. The combination of oceanographic data with suitable mathematical models represents a significant improvement in our ability to predict the statistical variability of ocean outfall plume behavior.